I am going to post here all newly submitted articles on the arXiv related to superconducting circuits. If your article has been accidentally forgotten, feel free to contact me
13
Okt
2020
High-speed calibration and characterization of superconducting quantum processors without qubit reset
To Characterize and calibrate quantum processing devices a large amount of measurement data has to be collected. Active qubit reset increases the speed at which data can be gathered
but requires additional hardware and/or calibration. The experimental apparatus can, however, be operated at elevated repetition rates without reset. In this case, the outcome of a first measurement serves as the initial state for the next experiment. Rol. et al. used this restless operation mode to accelerate the calibration of a single-qubit gate by measuring fixed-length sequences of Clifford gates which compose to X gates [Phys. Rev. Appl. 7, 041001 (2017)]. However, we find that, when measuring pulse sequences which compose to arbitrary operations, a distortion appears in the measured data. Here, we extend the restless methodology by showing how to efficiently analyze restless measurements and correct distortions to achieve an identical outcome and accuracy as compared to measurements in which the superconducting qubits are reset. This allows us to rapidly characterize and calibrate qubits. We illustrate our data collection and analysis method by measuring a Rabi oscillation at a 250 kHz repetition rate without any reset, for a qubit with a decay rate of 1/2πT1=3 kHz.
We also show that we can measure a single- and a two-qubit average gate fidelity with Randomized Benchmarking 20 and 8 times faster, respectively, than measurements that reset the qubits through T1-decay.
Listening to the quantum vacuum: a perspective on the dynamical Casimir effect
Modern quantum field theory has offered us a very intriguing picture of empty space. The vacuum state is no longer an inert, motionless state. We are instead dealing with an entity
teeming with fluctuations that continuously produce virtual particles popping in and out of existence. The dynamical Casimir effect is a paradigmatic phenomenon, whereby these particles are converted into real particles (photons) by changing the boundary conditions of the field. It was predicted 50 years ago by Gerald T. Moore and it took more than 40 years until the first experimental verification.
10
Okt
2020
Single-atom maser with engineered circuit for population inversion
We present a blueprint for a maser with a single three-level artificial atom. The artificial atom is a superconducting quantum system of a transmon layout coupled to two resonators.
The system is pumped via a two-photon process. To achieve a population inversion, we engineer the quantum system and optimize its parameters, particularly the relaxation via an auxiliary low-Q cavity coupled to a transition between two excited states. We show numerically that such a maser can operate both in the intermediate coupling regime with super-Poissonian photon statistics and in the strong coupling regime, where the statistics is sub-Poissonian. For the former, the maser exhibits thresholdless behavior and for the latter, there is a well-defined pumping threshold. An interesting side-effect of the auxiliary resonator is that it allows overcoming the photon blockage effect for the pump, which would otherwise prohibit reaching high photon population. Finally, we observe the bistability of the steady-state Wigner function and the self-quenching effect for some parameters.
09
Okt
2020
Single-shot number-resolved detection of microwave photons with error mitigation
Single-photon detectors are ubiquitous and integral components of photonic quantum cryptography, communication, and computation. Many applications, however, require not only detecting
the presence of any photons, but distinguishing the number present with a single shot. Here, we implement a single-shot, high-fidelity photon number-resolving detector of up to 15 microwave photons in a cavity-qubit circuit QED platform. This detector functions by measuring a series of generalized parity operators which make up the bits in the binary decomposition of the photon number. Our protocol consists of successive, independent measurements of each bit by entangling the ancilla with the cavity, then reading out and resetting the ancilla. Photon loss and ancilla readout errors can flip one or more bits, causing nontrivial errors in the outcome, but these errors have a traceable form which can be captured in a simple hidden Markov model. Relying on the independence of each bit measurement, we mitigate biases in the measurement result, showing good agreement with the predictions of the model. The mitigation improves the average total variation distance error of Fock states from 13.5% to 1.3%. We also show that the mitigation is efficiently scalable to an M-mode system provided that the errors are independent and sufficiently small. Our work motivates the development of new algorithms that utilize single-shot, high-fidelity PNR detectors.
08
Okt
2020
Automatic Post-selection by Ancillae Thermalisation
Tasks such as classification of data and determining the groundstate of a Hamiltonian cannot be carried out through purely unitary quantum evolution. Instead, the inherent non-unitarity
of the measurement process must be harnessed. Post-selection and its extensions provide a way to do this. However they make inefficient use of time resources — a typical computation might require O(2m) measurements over m qubits to reach a desired accuracy. We propose a method inspired by the eigenstate thermalisation hypothesis, that harnesses the induced non-linearity of measurement on a subsystem. Post-selection on m ancillae qubits is replaced with tracing out O(logϵ/log(1−p)) (where p is the probability of a successful measurement) to attain the same accuracy as the post-selection circuit. We demonstrate this scheme on the quantum perceptron and phase estimation algorithm. This method is particularly advantageous on current quantum computers involving superconducting circuits.
07
Okt
2020
Generating two continuous entangled microwave beams using a dc-biased Josephson junction
We show experimentally that a dc-biased Josephson junction in series with two microwave resonators emits entangled beams of microwaves leaking out of the resonators. In the absence
of a stationary phase reference for characterizing the entanglement of the outgoing beams, we measure second-order coherence functions for proving entanglement up to an emission rate of 2.5 billion photon pairs per second. The experimental results are found in quantitative agreement with theory, proving that the low frequency noise of the dc bias is the main limitation for the coherence time of the entangled beams. This agreement allows us to evaluate the entropy of entanglement of the resonators, and to identify the improvements that could bring this device closer to a useful bright source of entangled microwaves for quantum-technological applications.
06
Okt
2020
Photon-instanton collider implemented by a superconducting circuit
Instantons, spacetime-localized quantum field tunneling events, are ubiquitous in correlated condensed matter and high energy systems. However, their direct observation through collisions
with conventional particles has not been considered possible. We show how recent advance in circuit quantum electrodynamics, specifically, the realization of galvanic coupling of a transmon qubit to a high-impedance transmission line, allows the observation of inelastic collisions of single microwave photons with instantons (phase slips). We develop the formalism for calculating the photon-instanton cross section, which should be useful in other quantum field theoretical contexts. In particular, we show that the inelastic scattering probability can significantly exceed the effect of conventional Josephson quartic anharmonicity, and reach order unity values.
Spectroscopic observation of crossover from classical Duffing oscillator to Kerr parametric oscillator
We study microwave response of a Josephson parametric oscillator consisting of a superconducting transmission-line resonator with an embedded dc-SQUID. The dc-SQUID allows to control
the magnitude of a Kerr nonlinearity over the ranges where it is smaller or larger than the photon loss rate. Spectroscopy measurements reveal the change of the microwave response from a classical Duffing oscillator to a Kerr parametric oscillator in a single device. In the single-photon Kerr regime, we observe parametric oscillations with a well-defined phase of either 0 or π, whose probability can be controlled by an externally injected signal.
Photocount statistics of the Josephson parametric amplifier: a question of detection
Parametric amplifiers are known to squeeze the vacuum state of the electromagnetic field, which results in predictable statistics of the photocounts at their output. However, recent
theoretical work arXiv:1112.4159 predicts a very different statistical distribution for an amplifier based on a Josephson junction. We test the hypothesis experimentally and recover the expected squeezed vacuum statistics. We explain this discrepancy by showing theoretically how the photocount statistics is dictated by the detection process, from single mode (our experiment) to multimode, fully resolved in frequency (as in arXiv:1112.4159).
05
Okt
2020
Photon decay in circuit quantum electrodynamics
Light does not typically scatter light, as witnessed by the linearity of Maxwell’s equations. We constructed a superconducting circuit, in which microwave photons have well-defined
energy and momentum, but their lifetime is finite due to decay into lower energy photons. The inelastic photon-photon interaction originates from quantum phase-slip fluctuation in a single Josephson junction and has no analogs in quantum optics. Instead, the surprisingly high decay rate is explained by mapping the system to a Luttinger liquid containing an impurity. Our result connects circuit quantum electrodynamics to the topic of boundary quantum field theories in two dimensions, influential to both high-energy and condensed matter physics. The photon lifetime data is a rare example of a verified and useful quantum many-body simulation.